The present invention relates to an organic sol and a solid compound based on titanium oxide and an amphiphilic compound and their preparation processes.
Colloidal sol or dispersions of titanium oxide in organic media are known. However, the preparation processes of these sols are complex from an industrial point of view. A first class of processes uses compounds sensitive to water such as titanium alkoxides. Another class generally involves the preparation of an aqueous sol in the first instance and secondly bringing this aqueous sol into contact with an organic phase in order to transfer the titanium oxide into the organic phase. Such an operating method is not suitable for preparing sols in polar phases which are miscible with water. There is therefore a requirement for simpler processes which also provide access to sols with varying characteristics.
A subject of the present invention is to provide such processes and such organic sols.
For this purpose, the organic sol according to the invention is characterized in that it comprises titanium oxide particles; an organic liquid phase and at least one amphiphilic compound chosen from the polyoxyethylenated alkyl ether phosphates.
The invention also relates to a process for the preparation of such a sol which, according to a first variant, is characterized in that said amphiphilic compound and the organic liquid phase are mixed together, then the titanium oxide particles are dispersed in the mixture obtained. According to a second variant, the process is characterized in that a mixture of titanium oxide particles and at least one said amphiphilic compound are mixed together and said mixture is dispersed in the organic liquid phase.
The sols according to the invention have the advantage of being able to exist in a wide range of solvents, polar or non-polar solvents.
Other characteristics, details and advantages of the invention will appear more fully on reading the following description and the various concrete but non-limitative examples intended to illustrate it.
In the remainder of the description, the expression sol or colloidal dispersion of titanium oxide designates any system constituted by fine solid particles of colloidal dimensions based on titanium oxide suspended in a liquid phase, said types also being optionally able to contain residual quantities of linked or adsorbed ions such as for example chlorides, sulphates, nitrates, acetates, citrates, ammoniums or organic bases such as diethylamine. It should be noted that in such dispersions, the titanium may either be totally in the form of colloids or simultaneously in the form of ions and in the form of colloids.
The particles of the sol according to the invention can be based on titanium dioxide with a mainly anatase crystal structure. xe2x80x9cMainlyxe2x80x9d means that the content of the anatase titanium dioxide particles is higher than 50% by mass. Preferably, the particles have an anatase content greater than 80%. The degree of crystallization rate and the nature of the crystalline phase are measured by XR diffraction.
These particles can also be of rutle structure.
The average diameter of these particles in the sol is generally at most 250 nm, preferably at least 15 nm, even more preferentially between 20 and 70 nm. It is specified here that the average diameter of the particles or colloids must be understood as designating the average hydrodynamic diameter of the latter, and as determined by quasi-elastic diffusion of light according to the method described by Michael L. Mc Connell in the Journal Analytical Chemistry 53, no. 8, 1007 A, (1981).
The particles of the sol according to the invention generally have a BET specific surface area of at least 200 m2/g, preferably at least 250 m2/g.
By BET specific surface area is meant the specific surface area determined by nitrogen adsorption according to the ASTMD 3663-78 standard based on the BRUNAUER-EMMETT-TELLER method described in the periodical xe2x80x9cThe Journal of the American Societyxe2x80x9d, 60, 309 (1938). In order to measure the specific surface area of the particles according to the invention, when they are in the form of a dispersion, it is essential to follow the measurement protocol which consists of eliminating the liquid phase from the dispersion then drying the particles under vacuum at a temperature of 150xc2x0 C. for at least 4 hours.
According to two variants of the invention, the titanium oxide particles of the sol can comprise a coating. Before the following description of these two variants, it can be specified here that in both cases, the particles have an average diameter generally of at most 100 nm, preferably at least 25 nm, even more preferentially comprised between 50 and 70 nm. These coated particles also generally have a BET specific surface area of at least 70 m2/g, preferably at least 100 m2/g.
In the first or these variant, the titanium oxide particles are at least partially coated with a layer of at least one metallic or silicon oxide, hydroxide or oxyhydroxide. For this first variant, reference can be made to Patent Application EP-A-880564 the teaching of which relating to the description of the product and its method of preparation is incorporated here by way of reference. These metallic oxides, hydroxides or oxyhydroxides can in particular be chosen from SiO2, ZrO2, aluminium, zinc, titanium or tin oxides, hydroxides or oxyhydroxides in simple or mixed form. By mixed is understood a metallic compound based on at least two of the aforementioned elements (silicoaluminates, etc.).
In general, the ratio by weight of the metallic oxides, hydroxides or oxyhydroxides to the titanium dioxide is at most 60% by weight. This ratio is a function of the use for which the particles are intended. Preferably, when the particles are used in a cosmetic application, this ratio is 25%, even more preferentially at most 20%.
This quantity of metallic oxide, hydroxide or oxyhydroxide is measured on the particles in dispersion by X-ray fluorescence.
According to a particular embodiment of the invention, the particles are at least partially covered with a layer of silica and/or an aluminium oxide, hydroxide or oxyhydroxide in simple or mixed form.
According to another embodiment, the particles are covered with a layer of silica and aluminium hydroxide or oxyhydroxide with an SiO2 content of 30% by weight and an Al2O3 content of 15% by weight with respect to the titanium dioxide.
According to more particular embodiment, the particles are covered with a layer of silica and aluminium hydroxide or oxyhydroxide with an SiO2 content of 15% by weight and an Al2O3 content of 5% by weight with respect to the titanium dioxide.
According to the second of these variants, the titanium oxide particles are at least partially covered with a first layer of at least one compound of cerium and/or iron, and a second layer of least one metallic or silicon oxide, hydroxide or oxyhydroxide. For this second variant, reference can be made to Patent Application WO-A-98/01392 the teaching of which relating to the description of the product and its preparation method are incorporated here by way of reference.
The compounds present in the first aforementioned layer are precursors of cerium or iron oxide, i.e. they are thermally decomposable to cerium or iron oxide. They may be cerium or iron salts.
Particles covered with a cerium compound are preferred. The ratio by weight of the cerium compound(s) to the titanium dioxide is preferably at most 6% by weight, expressed in CeO2. This ratio can be optimized as a function of the size of the particles. Thus, It has been observed that for particles of 25 nm diameter, the optimum cerium content was 5.5% by weight, expressed in CeO2, with respect to the titanium dioxide, similarly for particles of 45 nm diameter, this content is 4.5%; for particles of 60 nm diameter, this content is 3%, and for particles of 80 nm diameter, this content is 2%.
The particles of this second variant are also at least partially covered with a second layer based on at least one metallic oxide, hydroxide or oxyhydroxide. The oxide is generally SiO2, while the metallic hydroxide or oxyhydroxide can in particular be chosen from the aluminium, zinc, titanium or tin hydroxides or oxyhydroxides in simple or mixed form (as defined above).
In general, the ratio by weight of the metallic oxides, hydroxides or oxyhydroxides to the titanium dioxide is at most 60% by weight. This ratio is a function of the use for which the particles are intended. Preferably, when the particles are used in a cosmetic application, this ratio is at most 25%, even more preferentially at most 20%.
These quantities of metallic compounds, oxides, hydroxides or oxyhydroxides are measured on the particles in dispersion by X-ray fluorescence.
According to a particular embodiment, the particles are covered at least partially with a first layer of a compound of cerium and a second layer based on silica and/or an aluminium hydroxide or oxyhydroxide in simple or mixed form. The contents by weight can in this case be 15% of SiO2 and 5% of Al2O3 with respect to the titanium dioxide. The second layer can also be based solely on silica with a content by weight of 30% of SiO2.
The organic liquid phase of the sol according to the invention can be based on an organic liquid or a very varied mixture of organic liquids.
The organic solvent or liquid can be an inert cycloaliphatic or aliphatic hydrocarbon, or a mixture of the two, such as for example mineral spirits or naphtha which may also contain aromatic components. There can be mentioned for example hexane, heptane, octane, nonane, decane, cyclohexane, cyclopentane, cycloheptane and liquid naphthenes. The aromatic solvents such as benzene, toluene, ethybenzone and the xylenes are also suitable as well as petroleum fractions of ISOPAR or SOLVESSO type (trade marks registered by the company EXXON), in particular SOLVESSO 100 which mainly contains a mixture of methylethyl and trimethylbenzene, and SOLVESSO 150 which contains a mixture of alkyl benzenes, in particular dimethylethylbenzene and tetramethylbenzene.
Chlorinated hydrocarbons can also be used such as chloro or dichlorobenzene, chlorotoluene as well as aliphatic and cycloaliphatic ethers such as diisopropyl ether, dibutyl ether and aliphatic and cycloaliphatic ketones such as methylisobutylketone, diisobutylketone, mesityl oxide.
Ketones can also be used such as acetone, aldehydes, nitrogenous solvents such as acetonitrile, alcohols, acids and phenols.
Esters can also be envisaged. As esters which may be used there can in particular be mentioned those resulting from the reaction of acids with C1 to C8 alcohols and in particular secondary alcohol palmitates such as isopropanol. The acids from which these esters originate can be aliphatic carboxylic acids, aliphatic sulphonic acids, aliphatic phosphonic acids, alkylarylsulphonic acids and alkylarylphosphonic acids having approximately 10 to approximately 40 carbon atoms, either natural or synthetic. By way of example there can be mentioned the fatty acids of tallol, coconut oil, soya oil, tallow oil, linseed oil, oleic acid, linoleic acid, stearic acid and its isomers, pelargonic acid, capric acid, lauric acid, myristic acid, dodecylbenzenesulphonic acid, 2 ethyl hexanoic acid, naphthenic acid, hexoic acid, toluene-sulphonic acid, toluene-phosphonic acid, lauryl-sulphonic acid, lauryl-phosphonic acid, palmityl-sulphonic acid and palmityl-phosphonic acid.
According to a particularly useful characteristic of the sol according to the invention, the organic liquid phase is based on a polar solvent or a mixture of polar solvents. By polar solvent is meant those having a dielectric constant ∈r greater than 5, as defined in particular in the publication xe2x80x9cSolvents and Solvent Effects in Organic Chemistryxe2x80x9d, C. Reichardt, VCH, 1988. This polar solvent can be chosen from halogenated solvents such as dichloromethane; esters of ethyl acetate, isopropyl palmitate, methoxy-propyl acetate type; alcohols such as ethanol, butanol or isopropanol; polyols such as propane diol, butane diol or diethylene glycol; ketones such as cyclohexanone or 1-methylpyrrolidin-2-one.
According to an important characteristic of the invention, the sol further comprises an amphiphilic compound. Without wishing to be bound by one explanation, it can be considered that this amphiphilic compound is adsorbed on or in electrostatic interaction with the titanium oxide particles or also complexed with the latter.
This compound is chosen from the polyoxyethylenated alkyl ether phosphates. The polyoxyethylenated alkyl ether phosphates of the following formula are meant here: 
or also the polyoxyethylenated dialkoyl phosphates of formula: 
in which R1, R2, R3, identical or different, represent a linear or branched alkyl radical, in particular with 2 to 20 carbon atoms; a phenyl radical; an alkylaryl radical, more particularly an alkylphenyl radical, in particular with an alkyl chain with 8 to 12 carbon atoms; an arylalkyl radical, more particularly a phenylaryl radical; n the number of ethylene oxide being able to vary from 2 to 12 for example; M1 represents a hydrogen, sodium or potassium atom. The R3 radical can in particular be a hexyl, octyl, decyl, dodecyl, oleyl, nonylphenyl radical.
As an example of this type of amphiphilic compound there can be mentioned those marketed under the brand names Lubrophos(copyright) and Rhodafac(copyright) and in particular the products below:
the poly-oxy-ethylene-alkyl (C8-C10) ether phosphates Rhodafac(copyright) RA 600
the polyoxyethylene tri-decyl ether phosphate Rhodafac(copyright) RS 710 or RS 410
the poly-oxy-ethylene oleoketyl ether phosphate Rhodafac(copyright) PA 35
the poly-oxy-ethylene nonylphenyl ether phosphate Rhodafac(copyright) PA 17
the poly-oxy-ethylene nonyl(branched) other phosphate Rhodafac(copyright) RE 610
The amphiphilic compound is chosen as a function of the nature of the organic liquid phase. More specifically, this choice is made by adapting the hydrophilic/lipophilic equilibrium of the amphiphilic compound to the hydrophilic/lipophilic character of the organic phase. In other words, the more polar the solvent contained in the organic phase, the more hydrophilic the amphiphilic compound will be.
The proportion of amphiphilic compound with respect to the titanium oxide is generally comprises between 2 and 10 molecules per nm2 of titanium oxide surface area, assuming a surface area comprised between 10 and 80 xc3x852 per complexing head of titanium cation.
The sol according to the invention have a concentration of titanium compound which can reach 40% expressed by weight of TiO2 with respect to the total weight of the dispersion.
The organic sole produced in this way have excellent stability. No settlement is observed after several months.
The present invention also relates to a solid compound which is characterized in that it comprises a mixture of titanium oxide particles and at least one amphiphilic compound chosen from those described above.
This solid compound is presented either in the form of a paste or in the form of a powder. The titanium oxide is presented in this solid compound in the form of aggregated elementary crystallites, the average size of the aggregates ranging from 20 to 100 nm. The sold compound has the property of being redispersible, i.e. being able to produce a sol according to the invention and as described above when it is suspended in an organic liquid phase.
The titanium oxide particles of the solid compound according to the invention can comprise a coating which is at least partial in the form of a layer of at east one silicon or metallic oxide, hydroxide or oxyhydroxide or also in the form of a first layer of at least one compound of cerium and/or iron and a second layer of at least one silicon or metallic oxide, hydroxide or oxyhydroxide.
What was described previously regarding the particles of the sol also applies here for the solid compound,
The preparation processes for the solid compound and the sol according to the invention will now be described.
As starting product, any titanium oxide is used which is capable of producing a sol when it is dispersed in a liquid phase and in particular any titanium oxide capable of being in the form described above regarding the solid compound.
There follows the description of and the preparation process for the titanium oxide particles which are particularly suitable as starting products for preparing the sol and the solid compound according to the invention.
The starting products are based on titanium dioxide with a mainly anatase crystal structure as defined previously.
These starting anatase titanium dioxide particles can be of a size of at most 100 nm, preferably at least 15 nm, yet more preferentially comprised between 20 and 70 nm. This diameter is measured by transmission electron microscopy (TEM). Their BET specific surface area is generally at least 200 m2/g, preferably at least 250 m2/g. This BET specific surface area is measured as defined previously.
The starting particles also have a density of the order of 2.5. By xe2x80x9cof the order ofxe2x80x9d is meant that the density is 2.5xc2x10.2. This density is given by the following formula:   density  =            1                        (                      1            /            ρ                    )                +        Vi              .  
in which:
xcfx81 is the density of the anatase, i.e. 3.8,
VI is the volume provided by the intraparticle pores, measured by the BJH method. By volume measured by the BJH method is meant the volume measured using the BARRETT-JOYNER-HELENDA method described in an article in the publication xe2x80x9cTechniques de l""Ingxc3xa9nleurxe2x80x9d and entitled xe2x80x9cTexture des solides poreux ou divsxc3xa9sxe2x80x9d, p. 3645-1 to 3645-13.
In order to measure the volume provided by the intraparticle pores of the particles according to the invention, when they are in the form of a dispersion, it is essential to follow the measurement protocol which consists of eliminating the liquid phase of the dispersion then drying the particles under vacuum at a temperature of 150xc2x0 C. for at least 4 hours.
Particles as described above can be obtained by hydrolysis of at least one titanium compound A in the presence of at least one compound B chosen from:
(i) the acids having;
either a carboxyl group and at least two hydroxyl and/or amine groups.
or at least two carboxyl groups and at least one hydroxyl and/or amine group,
(ii) the organic phosphoric adds of the following formulae: 
in which n and m are integers comprised between 1 and 6, p is an integer comprised between 0 and 5, R4, R5 and R6 identical or different represent a hydroxyl, amino, aralkyl, aryl, alkyl group or hydrogen,
(iii) the compounds capable of releasing sulphate ions in an acid medium,
(iv) the salts of the acids described above, and in the presence of anatase titanium dioxide seeds.
The starting solution, intended to be hydrolyzed, is preferably totally aqueous; another solvent, such as an alcohol, can optionally be added provided that the titanium compound A and compound B used are substantially soluble in this mixture.
As regards the titanium compound A, a compound chosen from titanium halides, oxyhalides, alkoxides, sulphates and more particularly synthetic sulphates is generally used.
By synthetic sulphates is meant solutions of titanyl sulphates produced by ion exchange from very pure titanium chloride solutions or by reacting sulphuric acid with a titanium alkoxide.
The preparation is preferably carried out with titanium compounds of the titanium halide or oxyhalide type. The titanium halides or oxyhalides which are more particularly used in the present invention are titanium fluorides, chlorides, bromides and iodides (respectively oxyfluorides, oxychlorides, oxybromides and oxyiodides).
According to particularly preferred method, the titanium compound is titanium oxychloride TiOCl2.
The quantity of the titanium compound A present in the solution to be hydrolyzed is not critical.
The initial solution further contains at least one compound B as defined previously. As non-limitative example of compounds B falling within the scope of the present invention, there can be mentioned in particular:
hydroxypolycarboxylic acids, and more particularly hydroxydi- or hydroxytricarboxylic acids such as citric acid, maleic acid and tartaric acid,
(polyhydroxy)monocarboxylic acids, such as for example glucoheptonic acid and gluconic acid,
poly(hydroxycarboxylic) acids, such as for example tartaric acid
dicarboxylic monoacids and their corresponding amides, such as for example aspartic acid, asparagine and glutamic acid,
monocarboxylic aminoacids, hydroxylated or non-hydroxylated, such as for example lysine, serine and threonine,
methylene aminotriphosphonate, methylene ethylenediaminotetraphosphonate, methylene triethylenetetraaminohexaphosphonate, methylene tetraethylenepentaaminoheptaphosphonate, methylene pentaethylenehexaaminooctaphosphonate,
methylene diphosphonate; 1,1xe2x80x2 ethylene diphosphonate; 1,2 ethylene diphosphonate; 1,1xe2x80x2 propylene diphosphonate; 1,3 propylene diphosphonate; 1,6 hexamethylene diphosphonate; 2,4 dihydroxypentamethylene 2,4 disphosphonate; 2,6 dihydroxyhexmethylene 2,5 diphosphonate; 2,3 dihydroxybutylene 2,3 diphosphonate; 1 hydroxybenzyl 1,1xe2x80x2 diphosphonate; 1 aminoethylene 1-1xe2x80x2 diphosphonate; hydroxymethylene diphosphonate; 1 hydroxyethylene 1,1xe2x80x2 diphosphonate; 1 hydroxypropylene 1-1xe2x80x2 diphosphonate; 1 hydroxybutylene 1-1xe2x80x2 diphosphonate; 1 hydroxyhexamethylene 1,1xe2x80x2 diphosphonate.
As already Indicated, it is also possible to use all the salts of the aforementioned acids as compound B. In particular, these salts are either alkaline salts and more particularly sodium salts, or ammonium salts.
These compounds can also be chosen from sulphuric acid and ammonium and in particular potassium sulphates.
Preferably, the compounds B as defined above are hydrocarbon-comprising compounds of aliphatic type. In this case, the length of the main hydrocarbon-comprising chain preferably does not exceed 15 carbon atoms, and more preferentially 10 carbon atoms. The preferred compound B is citric acid.
The quantity of compound B is not critical. In general, the molar concentration of compound B with respect to that of titanium compound A is comprised between 0.2 and 10% and preferably between 1 and 5%.
Finally, the starting solution comprises titanium dioxide seeds. The preferred specific characteristics of these seeds and their use are given below.
Thus, the titanium dioxide seeds used in the present invention firstly are of a size of less than 8 nm, measured by X-ray diffraction. Preferably, titanium dioxide seeds are used having a size comprised between 3 and 5 nm.
Next, the ratio by weight of titanium dioxide present in the titanium seeds present in the hydrolysis medium before introducing the seedsxe2x80x94i.e. provided by titanium compound Axe2x80x94and expressed in TIO2 is comprised between 0.01 and 3%. This ratio can preferentially be comprised between 0.05 and 1.5%. The combination of these two conditions with respect to the seeds (size and ratio by weight) combined with the process described previously allows precise control over the final size of the titanium dioxide particles, a level of seeds being associated with a particle size. Particles can be obtained in this way with a diameter which varies between 20 and 100 nm.
Titanium dioxide seeds are used in anatase form in order to induce the precipitation of the titanium dioxide in anatase form. Generally, as a result of their small size, these seeds are in a poorly crystallized anatase form. These seeds usually take the form of an aqueous suspension constituted by titanium dioxide. They can generally be obtained in a known manner by a process for neutralizing a titanium salt by a base.
The following stage consists in hydrolyzing the starting solution by any means known to a person skilled in the art and in general by heating. In the latter case, hydrolysis can preferably be carried out at a temperature greater than or equal to 70xc2x0 C. It is also possible initially to operate at a temperature lower than the medium""s boiling point then maintain the hydrolysis medium level at the boiling point.
Once hydrolysis is achieved, the titanium dioxide particles obtained are recovered by separating the precipitated solid from the mother liquors. They can be redispersed in a liquid medium, preferably acid or basic, for example in water, in order to obtain a dispersion of titanium dioxide.
According to a variant of the process, after recovering the particles obtained following hydrolysis and before they are redispersed, the particles can be neutralized and/or subjected to at least one washing operation. The particles can be recovered for example by centrifuging the solution resulting from hydrolysis, they are then neutralized by a base, for example an ammonium hydroxide or soda solution. They can be washed by redispersing them in an aqueous solution then the particles are separated from the aqueous washing phase. After optionally one or more other washing operations of the same type, the particles are redispersed in a liquid, for example water, which can preferably be acid or basic.
As regards titanium oxide particles in rule form, these can be obtained by hydrolysis of a titanium compound chosen from the titanium halides, oxyhalides and alkoxides.
Titanium particles can be used in solid form by simple evaporation or by drying at a temperature of at most 120xc2x0 C. of the aforementioned dispersion, i.e. that obtained after hydrolysis, separation from the hydrolysis medium and replacing in a liquid phase. This temperature is preferably comprised between 30 and 80xc2x0 C.
As regards the preparation of the titanium oxide particles according to the two variants described above, i.e. those in which the particles comprise a coating, the process is carried out according to the contents of the aforementioned Patent Applications EP-A-880584 and WO-A98/01392. It can be recalled here, in the case of the first variant, that the process consists of precipitating at least one metallic oxide, hydroxide or oxyhydroxide on the surface of the titanium dioxide particles. This precipitation can be achieved by introducing into a dispersion of precursors of titanium dioxide particles metallic oxides, hydroxides or oxyhydroxides generally in the form of aqueous solutions of salts, then modifying the pH in order to obtain the precipitation of these oxides, hydroxides or oxyhydroxides on the titanium dioxide particles.
In the case of the second variant, the process consists of precipitating at least one compound of cerium and/or iron on the surface of the titanium dioxide particles then of precipitating at least one metallic oxide, hydroxide or oxyhydroxide on the surface of the particles obtained.
These precipitations can also be achieved by introducing into a dispersion of precursors of titanium dioxide particles compounds of cerium and/or iron, metallic oxides, hydroxides or oxyhydroxides generally in the form of aqueous solutions of salts, then modifying the pH in order to obtain the precipitation of these compounds, oxides, hydroxides or oxyhydroxide on the titanium dioxide particles.
In general and for the two variants, this precipitation is carried out at a temperature of at least 50xc2x0 C.
The preparation process for the sol according to the invention can be implemented according to a first variant in this variant the aforementioned amphiphilic compound and the organic liquid phase are mixed together, then the titanium oxide particles are dispersed in the mixture obtained. It should be noted that it is possible either to introduce the solid particles into the amphiphilic compound/organic phase mixture or to pour this mixture onto the titanium oxide particles. Once the particles, the amphiphilic compound and the organic phase have been brought into contact, agitation is carried out until a stable colloidal dispersion is obtained.
There is a second variant of the process. In this case, a mixture of titanium oxide and at least one aforementioned amphiphilic compound is formed. This mixture can be achieved using any known mechanical means such as mixing in order to obtain a homogeneous paste. In this way, a solid compound is obtained as defined above. Said mixture is then dispersed in the organic liquid phase.
A third variant will now be described which is more particularly suitable for the preparation of a sol in a polar organic phase.
This variant, for the preparation of a sol according to the invention in an organic liquid phase (a) includes a first stage in which a dispersion is formed comprising titanium oxide particles and at least one amphiphilic compound of the aforementioned type in an organic liquid phase (b) based on a solvent with lower polarity than that of the solvent of the organic liquid phase (a). On formation of this dispersion, segregation can be observed due to water which may be present in the starting hydrated titanium oxide. In this case, the segregated water is separated from the rest of the dispersion.
In a second stage, the solid phase of the dispersion is separated from its liquid phase by). This separation can be carried out by any suitable technique. The separation can thus be carried out by flocculation by a third solvent or also by distillation or evaporation. Following this separation, a solid phase is obtained which can be dried and which takes the form, depending on the extent of drying, either of a powder or of a paste and which constitutes a solid compound according to the invention. In a last stage, the phase or the solid compound obtained in this way is redispersed in the organic phase (a) in order to obtain the sought sol.
The dispersions of titanium oxide particles in the organic phase can be subjected to an ultrafiltration treatment in order to improve their stability if necessary.
Finally, it should be noted that the sols obtained can be subjected to a dehydration post-treatment by passing them over a solid desiccant for example.
The sols according to the invention can be used in all the applications where titanium is used for its photocatalytic properties. In this case, the titanium oxide particles do not comprise any coating of the type described above.
The sols based on titanium oxide particles according to the invention and in particular those comprising particles with a coating of this type can be used as anti-UV agents in the preparation of formulations for cosmetics, varnishes, paints and plastics.
Non-limitative examples will now be given.